1. Field of the Invention
The present invention relates to a top gate thin film transistor (top gate type thin film transistor) that uses an oxide semiconductor for a semiconductor layer, and a display apparatus including the top gate thin film transistor.
2. Description of the Related Art
Thin film transistors (TFTs) that use amorphous silicon or low-temperature polysilicon for a semiconductor layer are widely used in a drive circuit in display apparatuses such as active matrix liquid crystal display devices and organic electroluminescence (EL) display devices. However, the manufacture of those TFTs requires a high temperature process, and hence it is difficult to employ a flexible substrate such as a plastic substrate or a film substrate which is low in heat resistance.
Meanwhile, in recent years, the development of TFTs that use, for a semiconductor layer, an oxide semiconductor having ZnO as a main component has recently become active. This oxide semiconductor can be made into a film at low temperature, and hence it is possible to form a TFT on a plastic substrate, a film substrate, or the like. However, the oxide semiconductor having ZnO as the main component has high sensitivity with respect to an atmosphere, and hence, in order to enable actual use of the TFT, it is necessary to isolate the oxide semiconductor layer from the atmosphere by a protective layer.
A plasma-enhanced chemical vapor deposition (PECVD) method is generally used as a method of forming the protective layer, which is high in film formation speed and is excellent in productivity. However, it is known that an insulating layer made of silicon nitride or the like, which is formed by a PECVD method and serves as a protective layer, has a large hydrogen amount in the layer, and hence when hydrogen is diffused from the protective layer into a channel region formed in the oxide semiconductor, the resistance of the oxide semiconductor is reduced (see SID'08 Digest, p. 633 (2008)).
As a method of preventing hydrogen from diffusing into the channel region, there is disclosed a method of forming a protective layer containing hydrogen while using a gate insulating layer and a gate electrode layer as a mask in a top gate polycrystalline oxide TFT having ZnO as the main component (see Japanese Patent Application Laid-Open No. 2007-220817). In this case, the hydrogen concentration of the semiconductor layer in an unmasked region increases, and therefore the resistance of the unmasked region of the semiconductor layer reduces. In this manner, source and drain electrode layers are formed in a self-alignment manner, and thus a TFT having a coplanar structure may be obtained.
In the structure disclosed in Japanese Patent Application Laid-Open No. 2007-220817, in order to prevent hydrogen from diffusing into the channel region of the semiconductor layer when the protective layer containing hydrogen is formed, the gate insulating layer or the gate electrode layer to be the mask is required to have a function of sufficiently suppressing the hydrogen diffusion.
When silicon oxide or the like is used for the gate insulating layer, in order to suppress reduction in resistance of the semiconductor layer, the gate insulating layer is preferred to be formed thick. However, the increase in thickness of the gate insulating layer may cause an increase in drive voltage of the TFT, or lowering of an on-current or deterioration of an S value.
Therefore, it is more desired to provide a function of suppressing hydrogen diffusion to the gate electrode layer. However, the inventors of the prevent invention have been made clear that, when Mo is used for the gate electrode layer, hydrogen diffusion cannot be suppressed even if the film thickness is 200 nm. Further, even when a polycrystalline oxide semiconductor such as indium tin oxide (ITO) and ZnO is used for the gate electrode layer, hydrogen is diffused through grain boundaries in the polycrystalline oxide semiconductor, and hence the reduction in resistance of the channel region of the semiconductor layer cannot be suppressed. From the facts above, a gate electrode layer capable of suppressing hydrogen diffusion has been desired.